1. Field of the Invention
This invention relates to lighting arrangements and light emitting devices that use LED-based (Light Emitting Diode-based) light sources to excite a photoluminescence material, typically a phosphor, to generate a desired color of light. In particular, although not exclusively, the invention concerns lighting arrangements that use a photoluminescence wavelength conversion component that is positioned remotely to the light source. More particularly the invention concerns LED-based light sources for use in such lighting arrangements and devices.
2. Description of the Related Art
White light emitting LEDs (“white LEDs”) are known and are a relatively recent innovation. It was not until LEDs emitting in the blue/ultraviolet part of the electromagnetic spectrum were developed that it became practical to develop white light sources based on LEDs. As taught, for example in U.S. Pat. No. 5,998,925, white LEDs include one or more phosphor materials, that is photoluminescence materials, which absorb a portion of the radiation emitted by the LED and re-emit light of a different color (wavelength). Typically, the LED chip or die generates blue light and the phosphor(s) absorbs a percentage of the blue light and re-emits yellow light or a combination of green and red light, green and yellow light, green and orange or yellow and red light. The portion of the blue light generated by the LED that is not absorbed by the phosphor material combined with the light emitted by the phosphor provides light which appears to the eye as being nearly white in color.
Due to their long operating life expectancy (>50,000 hours) and high luminous efficacy (70 lumens per watt and higher) high brightness white LEDs are increasingly being used to replace conventional fluorescent, compact fluorescent and incandescent light sources.
Typically in white LEDs the phosphor material is mixed with a light transmissive material such as a silicone or epoxy material and the mixture applied directly to the light emitting surface of the LED die. It is also known to provide the phosphor material as a layer on, or incorporate the phosphor material within, an optical component (a photoluminescence wavelength conversion component) that is located remotely to the LED die(s). Advantages of providing the phosphor remote to the excitation source are a reduced likelihood of thermal degradation of the phosphor material and a more consistent color of generated light.
The inventors have discovered that the dominant wavelength of the excitation light (typically blue) used to excite the phosphor can have a significant effect on the color and/or color temperature of the light emitted by the arrangement/device. For example for a 3000K white light emitting arrangement a variation of 2.5 nm in the excitation wavelength results in a color shift of about one MacAdam ellipse in the output of the arrangement. The present invention arose in an endeavor to provide an LED-based light source for use with a photoluminescence wavelength conversion component that at least in part overcomes the limitations of the known sources.